ABSTRACT

Dispersion of ground-water solutes was investigated as part of a multispecies
reactive tracer test conducted under spatially varying chemical conditions
in an unconfined, sewage-contaminated, sand-and-gravel aquifer on Cape Cod,
Massachusetts. The fate and transport of two tracers, nonreactive bromide
and reactive nickel (complexed with ethylenediaminetetraacetic acid (EDTA)),
are reported here. About 14 percent of the aqueous nickel mass was lost throughout
the test, probably due to reversible adsorption of the nickel-EDTA complex
onto the iron and aluminum oxyhydroxide minerals coating the aquifer sediments.
The two tracers traveled through the aquifer at constant velocities, but the
nickel was retarded relative to the bromide (retardation factor, 1.2). The
tracers showed little dispersion transverse to the direction of flow (vertical
dispersivity, 0.5-3.8 millimeters; lateral dispersivity, 1.4-1.5 centimeters)
and great dispersion in the direction of flow (longitudinal dispersivity,
1.1-2.2 meters). The nickel tracer cloud showed smaller longitudinal dispersion
than the bromide cloud, possibly due to nonlinear adsorption that led to proportionally
greater removal of mass at the edges of the tracer cloud where concentrations
were lower. This smaller dispersion also may have resulted from greater adsorption
in the shallow uncontaminated part of the aquifer that coincidentally had
a higher flow velocity than the deep part of the aquifer. The bromide dispersion
results are similar to those obtained in a test conducted previously in this
aquifer. The similarity suggests that physical dispersion in this aquifer
was spatially stationary at the scale of about 300 meters covered by these
two overlapping tests.